In portable terminals such as cellular phones, an output amplifier having a two-stage configuration using, for example, bipolar transistors is used. In conventional cellular phones, high efficiency operation is needed in the whole output power range. Therefore, bias points of the transistors of both the front stage and rear stage are set near the class B or class C. On the other hand, in cellular phones of the next generation or the third generation, its output power must be controlled according to the distance from the base station. Therefore, the output dynamic range is very wide as compared with the conventional technique, and high efficiency operation is demanded in the whole output power range thereof. Furthermore, there is a demand for suppression of adjacent channel leak power to a low value. For satisfying the demand, it is necessary to suppress the amplifier distortion, which becomes the major cause of the adjacent channel leak power, to a low value.
FIG. 1 is a circuit diagram showing principal parts of a conventional output amplifier. This amplifier has a two-stage configuration using bipolar transistors. This amplifier includes the input matching circuit 3 having the inductor 1 and capacitor 2, common emitter front stage transistor 4, inter-stage matching circuit 8 having the capacitors 5 and 7 and the inductor 6, common emitter rear stage transistor 9, and a not shown output matching circuit.
Base bias Vb1 of the front stage transistor 4 is supplied from the outside via an inductor 10. Collector bias Vcc1 of the front stage transistor 4 is supplied from the outside via an inductor 11. Base bias Vb2 of the rear stage transistor 9 is supplied from the outside via an inductor 12. Collector bias Vcc2 of the rear stage transistor 9 is supplied from the outside via an inductor 13. When this amplifier is used in a cellular phone or the like as described above, bias points of both the front stage transistor 4 and the rear stage transistor 9 are set near the class B or class C in order to implement high efficiency operation in the whole output power range.
RF signal (RFin) supplied from the outside is input to the base of the front stage transistor 4 via the input matching circuit 3. The collector output of the front stage transistor 4 is supplied to the base of the rear stage transistor 9 via the inter-stage matching circuit 8. RF signal (RFout) supplied from the collector of the rear stage transistor 9 is output to the outside via the output matching circuit. Numeral 14 denotes an RF signal input terminal, 15 denotes an RF signal output terminal, 16 denotes an application terminal of the base bias Vb1, 17 denotes an application terminal of the collector bias Vcc1, 18 denotes an application terminal of the base bias Vb2, and 19 denotes an application terminal of the collector bias Vcc2.
For making the conventional amplifier with a high efficiency over the whole output power range of the output dynamic range that is wider as compared with the conventional technique, it is necessary to set the bias points of the front stage transistor 4 and the rear stage transistor 9 at those of the class B or class C. By doing so, however, a gain change occurs as the output increases, resulting in a degraded distortion characteristic of the amplifier. On the other hand, for making the amplifier operate with low distortion, it is necessary to set the bias points at those of the class AB. In that case, however, the efficiency becomes lower when the output power is low or medium. Therefore, it is difficult to use the conventional amplifier in a cellular phone or the like of the next generation or the third generation, which demands both high efficiency operation and low distortion operation in a wide output dynamic range.